The water injection method used in oil recovery is where water is injected into the reservoir, usually to increase pressure and thereby stimulate production. Water is injected for two reasons: 1. For pressure support of the reservoir (also known as voidage replacement); 2. To sweep or displace the oil from the reservoir and push it towards an oil production well. Only 5 to 20% of the oil in a reservoir can be typically extracted by natural driving forces in the reservoir, but water injection increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period of time.
However, sweep recovery is limited by the so-called “thief zones,” whereby water preferentially travels through the more permeable regions or fractured zones of the reservoirs, bypassing the less permeable zones and leaving unswept oil behind.
One means of further improving recovery, therefore, is to partially block thief zones with a polymer gel or other material, thus forcing water through the less permeable regions.
U.S. Pat. Nos. 6,454,003, 6,984,705 and 7,300,973 describe an expandable crosslinked polymeric particle having an average particle diameter of about 0.05 to 10 microns (nano- to microparticle sizes). The particle is highly crosslinked with two crosslinkers, one that is stable and a second that is labile. The excess crosslinking makes the initial particles quite small, allowing efficient propagation through the pores of a reservoir. On heating to reservoir temperature and/or at a predetermined pH or other stimuli, the reversible (labile) internal crosslinks break, allowing the particle to greatly expand by absorbing additional injection fluid, usually water. The initial polymer is sometimes called the “kernel” before its expansion, in analogy to the way a kernel of popcorn “pops” in response to certain stimuli, such as heat.
The unique properties of this expandable particle allow it to fill the high permeability zones and then be expanded in situ so that the swollen particle blocks the thief zones and subsequent injections of fluid are forced to enter the remainder of the reservoir, more effectively sweeping the reservoir.
However, the method is limited in practice because subsequent water injections always remove some of the polymer. Hence the polymer becomes washed out and again presents the problem of allowing the injection fluid to avoid the less permeable zones.
The reason for the washout is not certain, but our own research suggested that the swollen polymer is not in a gel form, thus although viscous, is still a liquid that can be washed out of the porous substrate.
To address this problem, we have previously suggested a number of gelling techniques that serve to stabilize the polymer in situ, making it resistant to washout. See e.g., Ser. No. 12/722,344, filed Mar. 11, 2010 and claiming priority to 61/159,486, filed Mar. 12, 2009; Ser. No. 12/780,792, filed May 14, 2010 and claiming prior to 61/178,768 filed May 15, 2009; Ser. No. 12/815,314 filed Jun. 14, 2010 and claiming priority to 61/186,957 filed Jun. 15, 2009.
However, these patent applications address gelation, and not gelation rate. Thus, one additional problem is that gelation can occur too quickly, thus preventing the polymer from even reaching the deepest zones before it gels.
There are, however, available techniques to slow the gelation rate. For example, complexed multivalent cations such as chromium (III) acetate have been used as crosslinking or gelation agents to gel partially hydrolyzed polyacrylamides (HPAM) as described in U.S. Pat. No. 4,683,949. The delay occurs because of the time required for the complex to dissociate, thus releasing the cations for the gelation reactions to occur. See also U.S. Pat. Nos. 4,644,073 and 4,986,356.
While the complexed multivalent metal crosslinkers described above produce gels at much slower rates than Cr(III) chloride, the rate of gelation is still much too fast for placement of gelant deep into the oil-bearing formations. Thus, what is needed in the art, is a way to further delay the crosslinking of a polymer to allow it to fully penetrate the thief zones before gelation.